If you can get your hands on From Samarkhand to Sardis it thoroughly explains the deification of Seleukos. Basically, what seems to have happened was that Antiochos I spread the myth about his father in order to establish a solid dynasty - and to do so so as to stand out from the other successors and Alexander. Whether they existed before Seleukos's death or not I do not believe is certain.

For a close analogy, look to what Augustus did to establish his dynasty by making sure Caesar was deified and then have the position of emperor worshiped.